The present invention relates to mercury-probe test apparatus and a method of its operation. Such test apparatus is widely used for testing semi-conductor wafers, being exemplified in U.S. Pat. Nos. 3,794,912 issued Feb. 26, 1974 to P. J. W. Severin et al, and 4,101,830 issued July 18, 1978 to John H. Greig.
A wafer to be tested is positioned against an aperture plate in such test apparatus and mercury is displaced along a passage into contact with the wafer. In the course of repeated test cycles, bits of dirt tend to accumulate at the contact end of the mercury in the passage, degrading the tests. In order to clear the mercury in the passage of dross, it has been customary to expel some of the mercury, either at intervals or before each test. This involves a separate preparatory operation. Moreover, special care is needed in handling the expelled mercury to avoid the hazard of scattered mercury.
A variety of tests can be performed with mercury probes, each requiring an appropriate number of contacts. Where the tests involve capacitance measurements, the stray capacitance of the test apparatus must be taken into account. In bridge circuits, small values of stray capacitance can be "zeroed out" without appreciably affecting the test accuracy. A large amount of stray capacitance tends to interfere with the test accuracy. The reservoir represents a large contributor to the stray capacitance in apparatus where it remains connected to the wafer-engaging mercury, for example the apparatus shown in the Greig patent mentioned above. The problem is compounded where there are plural mercury-probe contacts each connected to its own mercury reservoir. There, the stray capacitance between reservoirs tends to be especially large.